1. Field of the Invention
The present invention relates to a method for preparing 1,5-dimethyltetralin using a dealuminated zeolite beta catalyst.
2. Description of the Related Art
1,5-dimethyltetralin (hereinafter referred to as “1,5-DMT”) is an intermediate raw material necessary for preparation of polyethylene naphthalate (PEN) and is prepared by the cyclization reaction of 5-ortho-tolylpentene (5-OTP), in particular, 5-(ortho-tolyl)-1-pentene, 5-(ortho-tolyl)-2-pentene or a mixture thereof.
1,5-DMT is subjected to dehydrogenation to form 1,5-dimethylnaphthalene (1,5-DMN) and this 1,5-dimethylnaphthalene is subjected to isomerization to form 2,6-dimethylnaphthalene (2,6-DMN). The isomerization is an equilibrium reaction, which gives 2,6-DMN consisting of an equilibrium composition of 1,6-DMN and 1,5-DMN as isomers thereof, and this 2,6-DMN is then subjected to a purification process using crystallization, as a post-process, to prepare high purity 2,6-DMN. The above 2,6-DMN is subjected to oxidation and hydrogenation to prepare 2,6-naphthalenedicarboxylic acid (2,6-NDA) serving as a raw material for polyethylene naphthalate.
A general method for preparing 2,6-naphthalenedicarboxylic acid (2,6-NDA) from the above-mentioned 5-ortho-tolylpentene (5-OTP) is represented by the following reaction scheme 1:

Polyethylene naphthalate (PEN) can be used in the applications of a large capacity magnetic tape capable of recording for a long time, a heat resistant capacitor, a tire cord and a container for drinks because it is excellent in thermal, mechanical, chemical and electrical properties, as well as in physical properties such as water resistance and air permeation resistance, as compared with polyethylene terephthalate (PET) which is currently widely used as a general purpose resin, and thus the demands of the polyethylene naphthalate tend to gradually increase.
Therefore, many researches on the preparation of 1,5-DMT, which is an intermediate raw material of polyethylene naphthalate, have been conducted. However, the related art regarding the preparation of 1,5-DMT from 5-OTP has disadvantages that a large amount of impurities are formed and hence an additional process for separating the impurities is required. Examples of the impurities formed in this case include a dimethylnaphthalene (DMN) isomer, a dimethyltetralin (DMT) isomer, a dimer and a polymeric substance. The term “dimer” as used herein refers to a substance having a molecular weight of 320 includes a dimer of reactants or reaction products. Examples of the dimer include a dimer of 5-OTP and 1,5-DMT and a dimer of two molecules of 5-OTP.

Therefore, in order to prepare 1,5-DMT having high conversion and high purity from 5-OTP, various methods including the improvement of the preparation process and the introduction of a new catalyst have been researched.
U.S. Pat. No. 4,950,825 discloses a method for preparing 1,5-DMT by the cyclization reaction of 5-OTP, in which zeolite Y or zeolite Y having a noble metal impregnated thereon is used as a catalyst and impurities such as arylpentane, a DMN isomer such as 1,4-DMN and 1,7-DMN, a DMT isomer, a dimer and a polymeric substance are formed in an amount of about 10%. In particular, the above-mentioned process has disadvantages that a dimer and a polymeric substance are formed in an amount of 5% or more and hence the selectivity and the yield to 1,5-DMT are deteriorated and an additional separation process is required.
Further, U.S. Pat. No. 5,034,561 discloses a method for preparing 1,5-DMT by the cyclization reaction of 5-OTP, in which an ultrastable, Y-type zeolite (hereinafter referred to as a “USY”) catalyst is used as a catalyst and it further comprises a process that a distillation column is provided at the rear end of the reactor for the purpose of separating 1,5-DMT, and thus 1,5-DMT having an improved purity is obtained. U.S. Pat. Nos. 5,030,781 and 5,401,892 also disclose that the USY catalyst was used as a catalyst. Furthermore, U.S. Pat. No. 5,284,987 discloses that a zeolite catalyst such as mordenite, zeolite X, zeolite Y and ZSM-5 was used as the cyclization catalyst of 5-OTP. U.S. Pat. No. 6,504,069 B1 and European Patent No. 1 031 550 A1 disclose a method for preparing 1,5-DMT by introducing various new catalysts including a catalyst prepared by impregnating a carrier such as zeolite, silica-alumina, silica-zirconia and silica-titania with sulfuric acid or phosphoric acid, in order to attempt an improvement on the reactivity. However, these patents have disadvantages that the formation of impurities such as a DMT isomer, a DMN isomer and a polymeric substance were not decreased and the yield of 1,5-DMT was largely lowered when the reaction temperature was decreased so as to inhibit the formation of the impurities. Further, the impurities formed lead to a decrease in the catalyst selectivity and have significant adverse effects such as deterioration in catalyst activity and a decrease in catalyst life.
Korean Patent No. 10-0303246 discloses a method for selectively preparing 1,5-DMT by the cyclization reaction of 5-ortho-tolylpentene using a zeolite beta catalyst. However, this process exhibits a high conversion and a high selectivity to 1,5-DMT at the initial reaction, but has a problem that the catalyst life becomes reduced due to the significant deactivation of the catalyst and a problem that it is difficult to industrially apply this process with the use of the zeolite beta catalyst in the form of powders.
In general, it is known that the deterioration in catalyst activity and the decrease in catalyst life are caused by poisoning from a product and impurities in the reaction of a hydrocarbon compound [Appl. Catal. 63, 197 (1990)].
U.S. Pat. No. 5,396,008 and European Patent No. 0 582 108 B1 disclose a method for preparing 1,5-DMT by the cyclization reaction of 5-OTP with a high conversion and a high selectivity, and an improved catalyst life, in which the cyclization reaction of 5-OTP is carried out in gaseous state in the presence of a diluent by the use of a catalyst comprising a weakly acidic carrier such as activated carbon, silica, titania and zirconia and aluminosilicate impregnated thereon, in order to prevent the deactivation of the catalyst and the decrease in catalyst life. However, the method has disadvantages that it further comprises a process of mixing and extruding the carrier and the catalyst substance for industrially applicability and also has problems that process deviation conditions may occur caused by the diluent and it requires a high cost because the reaction is carried out at high temperatures so as to maintain the reaction system in a gaseous state.
On the other hand, according to the related art, it is known that the cyclization reaction of 5-OTP to 1,5-DMT using a zeolite beta catalyst takes place at two active sites. It is reported that an isomerization reaction and a polymer reaction take place at the acid site existing on the external surface of the catalyst to form DMTs other than 1,5-DMT and the cyclization reaction to 1,5-DMT takes place at the acid site existing on the internal surface of the catalyst [J. Catal., 207, 183 (2002)].
Therefore, it is expected that the removal of active sites existing on the external surface of the zeolite beta catalyst will have considerable influence on the improvement on catalyst life.
In this regard, the present inventors have studied on a method for preparing 1,5-dimethyltetralin which has a high conversion and a high selectivity, and allows suppression of catalyst deactivation and improvement on catalyst life, and have found that the cyclization reaction of 5-OTP is carried out using a zeolite beta catalyst in which the active sites existing on the external surface thereof are selectively removed by dealumination, to obtain a high conversion and a high selectivity of 1,5-dimethyltetralin and to suppress deactivation of the zeolite beta catalyst, thereby improving catalyst life. Thus, they have completed the present invention.